The code needs to be compatible with D2007 and D2009.
My Answer: Thanks to everyone who answered, I've gone with:
function ComputerName : String;
var
buffer: array[0..255] of char;
size: dword;
begin
size := 256;
if GetComputerName(buffer, size) then
Result := buffer
else
Result := ''
end;
The Windows API GetComputerName should work. It is defined in windows.pas.